This invention relates to electrical circuits which produce signals representative of the motion of an object under examination, and more particularly, to electrical circuits useful in fetal heart monitors.
It is known that the motion of an object can be determined by creating a beam of ultrasonic energy with an ultrasonic generator, or transducer, directing the beam at the object, and measuring the doppler shift in the ultrasonic signal reflected from the object and picked up by the transducer. Electrical circuits are used to analyze the received signal to determine the doppler shift caused by the motion of the object in the field of the transmitted ultrasonic beam. The signals from the circuit are useful in determining the rate, velocity and direction of motion of an object, e.g. a fetal heart. With respect to a fetal heart, the signals are particularly useful in the determination of heart rates and related information.
In practice a reflected, doppler-shifted signal is applied to the circuit. This signal has information that is proportional to the velocity of the motion of the heart in the path of the transmitted ultrasonic beam. In particular, the input signal has an upper sideband that is produced by motion of the fetal heart under study toward the receiving transducer, and a lower sideband produced by motion of the heart away from the transducer. The amplitude of the upper and lower sideband ultrasonic signals received by the transducer and created by the doppler effect, relate to the direction and speed of motion of the heart.
The electrical circuitry separates the upper and lower sidebands of the doppler-shifted ultrasound signal into two signals that correspond to motion toward and away from the transducer. The amplitudes of these signals are processed and utilized to analyze the fetal heart rate and condition.
Although in the prior art circuits, the amplitudes of the received ultrasonic signals are analyzed, there is also information contained in the frequency change of the ultrasonic doppler shifted signal. Thus, it would be advantageous, and an improvement over prior art electrical circuitry devices, to have a circuit which detects frequency domain information from the ultrasonic signals that have been doppler shifted by motion in the field of the ultrasonic beam.